The present invention relates to a low-pressure mercury vapor-filled discharge lamp, a luminaire and a display device having a reduced diameter and an improved efficiency.
An example of well-known conventional low-pressure mercury vapor-filled discharge lamps using a double tube is disclosed in Japanese Utility Model Publication No. 52932/1992. That publication discloses a low-pressure mercury vapor-filled discharge lamp including an elongated cylindrical glass inner tube and a glass outer tube coaxially encompassing the inner tube with a space therebetween, the inner tube and the outer tube supported by support members at the ends thereof. The space thermally insulates the inner tube from the outside air so that the decrease in the luminance efficiency can be held down to a minimum even under the conditions where both the electric power input to the lamp and the heat capacity of the lamp are small and the temperature in the environment is low.
According to the above configuration, however, it is difficult to hermetically connect the inner tube and the outer tube at a low cost, because a support member for such a purpose has to leave sufficient thermal insulation capability and wettability with respect to glass.
In a low-pressure mercury vapor-filled discharge lamp incorporated in a display device, such as a subsurface illuminator facing a side of a light conducting element, a further reduction in the diameter and a further increase in the luminance of the lamp is desired in order to increase its incidence efficiency with respect to the light conducting element. The conventional configuration described above specifies the outer diameter of its inner tube to be 6 mm or less. However, since its electrodes are hot cathodes, there are limitations in reducing the diameter of the tube. In practice, it is difficult to make the inner diameter of the inner tube less than 3 mm. Another problem occurs in cases where a lamp having a space between the inner and the outer tubes in the range of 1 and 10 mm is mounted on a light conducting plate. In those cases, the space between the inner tube and the light conducting plate is too wide, resulting in unfavorably increased light loss. The luminance of a low-pressure mercury vapor-filled discharge lamp may be increased by increasing power input to the lamp. However, the problem cannot be overcome simply by increasing the power input to the lamp, because doing so causes a decrease in the efficiency of the lamp.
Although a discharge lamp is normally lit at a high frequency of more than 10 kHz in order to increase the lamp efficiency, it is a known fact that the efficiency of a discharge lamp adapted to be lit at such a high frequency decreases to a certain extent when mounted on an apparatus. This decrease in efficiency, which is caused by current leakage, is most prominent when the outer diameter of the lamp is less than 8 mm.